Distributed travelling-wave mach-zehnder modulator driver

ABSTRACT

A distributed traveling-wave Mach-Zehnder modulator driver having a plurality of modulation stages that operate cooperatively (in-phase) to provide a signal suitable for use in a 100 Gb/s optical fiber transmitter at power levels that are compatible with conventional semiconductor devices and conventional semiconductor processing is described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. patentapplication Ser. No. 16/220,203, now allowed, filed Dec. 14, 2018, whichclaims priority to U.S. patent application Ser. No. 16/108,857, filedAug. 22, 2018, now U.S. Pat. No. 10,205,531 which claims priority to andbenefit of U.S. patent application Ser. No. 15/830,351, filed Dec. 4,2017, now U.S. Pat. No. 10,084,545, which claims priority from U.S.patent application Ser. No. 15/234,359, filed Aug. 11, 2016, now U.S.Pat. No. 9,853,738, which is a continuation-in-part of and claimspriority from U.S. patent application Ser. No. 14/618,989, filed Feb.10, 2015, issued as U.S. Pat. No. 9,559,779, which claims priority toU.S. Provisional Application No. 61/937,683 filed Feb. 10, 2014, each ofwhich is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to optical transmitters in general, andparticularly to an optical driver for a Gigabit/second transmitter.

BACKGROUND OF THE INVENTION

Optical interconnects offer promising solutions to data transmissionbottlenecks in supercomputers and in data-centers as well as otherapplications. Adopting higher channel data rates can greatly reduce thecomplexity in optical communication systems and/or further improveinterconnect capacity and density.

The most important requirement on the driver amplifier is the outputvoltage swing. The state-of-the-art driver amplifier in CMOS/BiCMOS canoutput 3 Vpp at 40 Gb/s, consuming 1.35 W DC power. See for example, C.Knochenhauer, J. Scheytt, and F. Ellinger, “A Compact, Low-Power40-GBit/s Modulator Driver With 6-V Differential Output Swing in 0.25 umSiGe BiCMOS,” Solid-State Circuits, IEEE Journal of, vol. 46, no. 5, pp.1137-1146, 2011.

At higher data rates it is difficult to maintain or improve theavailable drive voltage without substantial advances in the fabricationprocess. This trend is at odds with the increasingly higher drivevoltage required by modulators at higher speed.

There is a need for improved drivers for use in optical data handlingsystems.

SUMMARY OF THE INVENTION

Accordingly, the present disclosure relates to a distributed travelingwave modulator, comprising:

an optical input for receiving an input optical signal and splits theinput optical signal into first and second portions;

a first plurality of optical phase-shifters connected in seriesoptically coupled to the optical input for receiving the first portionof the input optical signal;

a second plurality of optical phase-shifters connected in seriesoptically coupled to the optical input for receiving the second portionof the input optical signal, respective pairs of the first and secondplurality of optical phase-shifters forming sequential modulators;

an optical output for recombining the first and second portions forminga modulated output optical signal;

a differential electrical data input connected to a data source;

a plurality of driver amplifier stages, each including a respectivedifferential driver amplifier input, a differential driver amplifieroutput, and a differential signal output connected to a respective oneof the sequential modulators, the differential driver amplifier input ofa first of said plurality of driver amplifier stages connected to thedifferential electrical data input; and

a plurality of delay/relay stages, each including a respectivedifferential delay/relay input connected to a differential driveramplifier output of a previous driver amplifier stage, and adifferential delay/relay output connected to a differential driveramplifier input of a following driver amplifier stage;

An optical delay of each optical phase-shifter along with opticalwaveguide wiring therebetween matches a delay between the driveramplifier stages so that modulations constructively add.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1A is a block diagram of a typical optical transmitter and receiverof the prior art.

FIG. 1B is a graph that illustrates the power scaling versus data ratefor prior art silicon traveling-wave modulators.

FIG. 2A is a circuit block diagram of a distributed traveling-waveMach-Zehnder (TWMZ) modulator driver that operates according toprinciples of the invention.

FIG. 2B is a schematic circuit diagram of a driver amplifier stage ofthe distributed traveling-wave Mach-Zehnder modulator driver of FIG. 2A.

FIG. 2C is a schematic circuit diagram of a delay/relay stage of thedistributed traveling-wave Mach-Zehnder modulator driver of FIG. 2A.

FIG. 2D is an image of a chip that embodies the distributedtraveling-wave Mach-Zehnder modulator driver of FIG. 2A. The chip has awidth of 1 mm and a length of 2.9 mm. DC bias is provided by thestructure at the right side of the chip.

FIG. 3A is a graph that illustrates the results of a distributed TWMZdriver post-layout simulation showing 100 Gb/s eye-diagrams at thedriver outputs.

FIG. 3B is a graph that illustrates the results of a distributed TWMZdriver post-layout simulation showing 100 Gb/s eye-diagrams after the SiTWMZ. The differential output is 2 Vpp at 25Ω impedance; data is shiftedby 13 picoseconds (ps) between each output stage.

DETAILED DESCRIPTION

As illustrated in FIG. 1A, the analog front-end in a typical prior artoptical transceiver includes a modulator driver and a transimpedanceamplifier that serve as interfaces between a high-speed optical channeland lower speed digital electronics. Data is provided to the MUX and ismodulated onto a laser carrier in an optical fiber using the driver andthe modulator. The modulated carrier travels down the fiber. At areceiver, a photodetector samples the optical carrier and atransimpedance amplifier and DEMUX provide an electrical output signalthat represents the data provided to the MUX.

FIG. 1B shows the expected voltage requirement vs. data rate. As shownin FIG. 1B, the required drive voltage for the prior art modulator at100 Gb/s is >6 Vpp (on each swing-end output), which is far from apractical voltage in existing CMOS/BiCMOS technology.

We describe systems and methods to provide ultra-high channel rate (40to 100 Gb/s) optical transmitters in silicon-based electronics andphotonics technology.

We have described another design of a traveling wave modulator in RanDing, Yang Liu, Qi Li, Yisu Yang, Yangjin Ma, Kishore Padmaraju, AndyEu-Jin Lim, Guo-Qiang Lo, Keren Bergman, Tom Baehr-Jones, and MichaelHochberg, “Design and characterization of a 30-GHz bandwidth low-powersilicon traveling-wave modulator,” Optics Communications (availableonline Feb. 7, 2014).

In various embodiments of the present invention, the followingassumptions are made: Cpn is 230 fF/mm, Rpn is 5.5 Ω-mm, Vπ Lπ is 2.0V-cm, device bandwidth is 70% data rate, a differential-drive geometryis used, and an equivalent of Vπ/3 swing generate acceptable opticalmodulation amplitude. As an example, we describe a distributed TWMZdriver that can be fabricated in a 130-nm SiGe BiCMOS process in orderto bridge the gap between the increasingly higher drive-voltage requiredby modulators and limited available driver output voltage swing fromelectronics at higher data rates.

FIG. 2A is a circuit block diagram of a distributed traveling-waveMach-Zehnder (TWMZ) modulator driver that operates according toprinciples of the invention. An optical input waveguide 220 receives anoptical signal that is to be modulated, and splits the signal in two,one portion of the signal passing through wave shifters 210, 212, 214and 216, and the other portion of the signal passing through waveshifters 210′, 212′, 214′ and 216′. In a preferred embodiment, theoptical signal is split into two portions having equal intensities. Inone embodiment, the wave shifter pairs (210, 210′), (212, 212′), (214,214′), and (216, 216′), are Mach-Zehnder interferometers with fixedoptical lengths to minimize power consumption and increase speed. Theoptical signals are recombined and exit the modulator at optical port240. In some embodiments, the drive circuitry which will now bedescribed is attached to the chip using flip-chip bump bonding,illustrated by bonding interface 230.

As shown in the circuit block diagram in FIG. 2A, the driver amplifiertakes 400 mV_(pp) input signals at each of the differential inputs In+and In−, and delays and amplifies the signals to four pairs ofdifferential outputs with 13 ps delay between each output stage. Eachoutput swings 1 Vpp single-ended (2 Vpp differential) on a 25Ωimpedance. The output is intentionally configured to be open-collectorto offer the flexibility to drive both 25Ω and 50Ω impedance TWMZsections (without and with near-end termination, respectively). Thetermination resistors are not illustrated in the schematic shown in FIG.2B. They could be introduced during the packaging step or they could bemonolithically integrated on the modulator side. The open-collectornature of the proposed driver enables the TWMZ sections to be designedto have different impedance, increasing the size of the optimizationspace and the number of possible configurations.

In the preferred embodiment of FIG. 2A, there are illustrated aplurality of N of optical phase shifter pairs, where N=4. In otherembodiments, one can use a different number N of optical phase shifterpairs, so long as N is greater than or equal to 2. In the embodimentshown, the distributed traveling-wave Mach-Zehnder (TWMZ) modulatordriver has four driver amplifier stages 250 (illustrated in greaterdetail in FIG. 2B) and three delay/relay stages 260 (illustrated ingreater detail in FIG. 2C).

In other embodiments, one can use other kinds of optical phase shiftersin place of the TWMZ, so long as the number of optical phase shifters isgreater than or equal to 2.

FIG. 2B and FIG. 2C are circuit diagrams that illustrate preferredembodiments of the driver amplifier stages 250 and the delay/relaystages 260 of FIG. 2A, respectively. The driver stage 250, shown in FIG.2B, starts with a pair of emitter followers 252 a and 252 b equippedwith termination resistors for efficient coupling to the data source orto the previous stage of the driver. The received signal is thenamplified using a differential pair 254 a and 254 b, and subsequentlybuffered again using emitter followers 256 a and 256 b. Finally, thesignal is split and applied to two open collector cascode output drivers258 a and 258 b, one driving the TWMZ segment, and one 259 a and 259 bamplifying the signal for the following stage of the driver. Eachmodulator includes a driver amplifier stage 250, which includes only asingle type of transistor to enable high-speed operation. The preferredtype is NPN bipolar transistor; however other possible transistorsinclude a PNP, a MOSFET (NFET or PFET, either one) or even a HEMT or apHEMT.

In one embodiment, the integration interface between silicon TWMZsections and the driver circuits is expected to be flip-chipbump-bonding. A 40 fF parasitic capacitance is assumed for each signalconnection. The optical delay of each TWMZ section plus opticalwaveguide wiring matches the delay between the amplifier stages so thatthe modulations constructively add. As an additional step to improve theperformance, we have incorporated pre-amplification in the driver outputto extend the length of TWMZ sections that can be driven at 100 Gb/s byabout 40%. The driver pre-amplifier stage 254 a and 254 b is shown inFIG. 2B immediately after the input emitter follower stage 252 a and 252b. Without the pre-amplifier 254 a and 254 b the overall driver 250would not be able to have the gain-bandwidth product required to achievethe target 100 Gb/s data rate, especially when driving a longer TWMZsection.

The example circuit described above consumes 1.5 W power overall. The DCbias structures illustrated on the right of the chip (FIG. 2D) controlthe on and off states of each main driver stage individually, which is auseful feature for testing before integration. The bias voltages Vtb andVtb_main of the driver section 250 shown in FIG. 2B are generatedseparately in the DC Bias structures shown in FIG. 2D. Each of thedriver sections 250 can then be enabled or disabled by turning thecorresponding bias voltages on and off. This increases the flexibilityof the driver 250, allowing integration with different types of TWMZs.For example, if the TWMZ has only three sections, the fourth section ofthe driver (the one providing outputs Out4+ and Out4− in FIG. 2D) can beshut down, reducing the overall power dissipation. The feature can alsobe useful in other scenarios. For example, if optical modulationresulting from the action of fewer stages of the driver is found to beadequate, the redundant stages can be shut down. In addition, slightvariation in the biasing voltages of individual stages can be used tooptimize delays, gains and the overall performance of the driver-TWMZsystem.

In the embodiment shown in FIG. 2D the chip or substrate is silicon. Inother embodiments, the substrate can be fabricated from a semiconductor,which may be different from a silicon or silicon-on-insulator wafer.

Post-layout simulations at 100 Gb/s is shown in FIG. 3A and FIG. 3B. TheTWMZ sections are modeled using an equivalent circuit model.Bump-bonding parasitics are taken into account. As one can see, similarelectrical eye quality is maintained in each stage output and this isachieved by scaling the transmission lines and device sizes in eachstage. The eye-diagrams at the end of the TWMZ (FIG. 3B) provide aconservative estimation of the optical eye-diagrams.

In the driving scheme used in the embodiment of FIG. 2A through FIG. 2D,the overall drive voltage requirement is linearly lowered byaccumulating modulation from four sections of TWMZ of 750 μm length,achieving an overall modulator length of 3 mm, which is similar to a 40Gb/s device illustrated in FIG. 1B. The present device provides apractical solution for a 100 Gb/s optical transmitter.

In operation, an optical wave (or an optical signal) to be modulated isexpected to be received at an input port such as 220, subjected to asuccession of N modulations performed by successive ones of a pluralityN a plurality N of optical phase-shifters connected in series connectionas N sequential modulators, where N is greater than or equal to 2, eachof the N−1 phase shifts after the first of the N phase shifts delayed bya time calculated to apply each of the N−1 phase shifts after the firstof the N phase shifts at a respective time when the optical signalpasses a respective one of the N−1 sequential modulators after the firstmodulator, and providing a modulated optical signal at an optical outputport, such as port 240.

The apparatus described above can be used for performing such opticalmodulation as just described.

Definitions

Unless otherwise explicitly recited herein, any reference to anelectronic signal or an electromagnetic signal (or their equivalents) isto be understood as referring to a non-volatile electronic signal or anon-volatile electromagnetic signal.

Theoretical Discussion

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. A distributed traveling wave modulator,comprising: an optical input for receiving an input optical signal andsplitting the input optical signal into first and second portions; afirst plurality of optical phase-shifters connected in series opticallycoupled to the optical input for receiving the first portion of theinput optical signal; a second plurality of optical phase-shiftersconnected in series optically coupled to the optical input for receivingthe second portion of the input optical signal, respective pairs of thefirst and second plurality of optical phase-shifters forming sequentialmodulators; an optical output for recombining the first and secondportions forming a modulated output optical signal; a differentialelectrical data input connected to a data source; a plurality of driveramplifier stages, each including a respective differential driveramplifier input, a differential driver amplifier output, and adifferential signal output connected to a respective one of thesequential modulators, the differential driver amplifier input of afirst of said plurality of driver amplifier stages connected to thedifferential electrical data input; and a plurality of delay/relaystages, each including a respective differential delay/relay inputconnected to a differential driver amplifier output of a previous driveramplifier stage, and a differential delay/relay output connected to adifferential driver amplifier input of a following driver amplifierstage; wherein an optical delay of each optical phase-shifter plusoptical waveguide wiring matches a delay between the driver amplifierstages so that modulations constructively add.
 2. The modulatoraccording to claim 1, wherein each output of each driver amplifier stageis configured to be an open-collector driver for driving modulatorsegments with different impedances.
 3. The modulator according to claim1, wherein each differential driver amplifier output of each driveramplifier stage is configured to comprise an open-collector cascodedriver for driving the sequential modulators with different selectivelyinterchangeable impedances.
 4. The modulator amplifier according toclaim 3, wherein each differential driver amplifier output of eachdriver amplifier stage is configured to be an open-collector driver fordriving both 25Ω and 50Ω impedance modulator segments.
 5. The modulatoraccording to claim 1, further comprising: a plurality of DC biasstructures, each DC bias structure configured to individually controlone of said plurality of delay/relay stages; wherein each DC biasstructures is capable of controlling an on state and an off state of arespective delay/relay stage for shutting down individual delay/relaystages.
 6. The modulator according to claim 5, wherein furthercomprising DC bias structures also capable of controlling an on stateand an off state of a respective driver amplifier stage for shuttingdown individual driver amplifier stages.
 7. The modulator according toclaim 1, further comprising: a plurality of DC bias structures, each DCbias structure configured to individually control one of said pluralityof driver amplifier stages; wherein each DC bias structures is capableof controlling an on state and an off state of a respective driveramplifier stage for shutting down individual driver amplifier stages. 8.The modulator according to claim 1, wherein each driver amplifier stageincludes only a single type of transistor to enable high-speedoperation.
 9. The modulator according to claim 1, wherein each of theplurality of first and second optical phase shifters comprise a fixedoptical length.
 10. The modulator according to claim 1, wherein each ofthe driver amplifier stages includes a pre-amplifier stage.
 11. Themodulator according to claim 1, wherein each pair of optical phaseshifters comprises a Mach-Zehnder interferometer modulator.
 12. Themodulator according to claim 1, wherein each of the first and secondportions have equal intensities.
 13. The modulator according to claim 1,wherein the plurality of driver amplifier stages comprises four, and thenumber of delay/relay stages comprises three.
 14. The modulatoraccording to claim 1, wherein each driver amplifier stage includes: afirst pair of emitter followers including termination resistorsconnected to the differential driver amplifier input; a pre-amplifiercomprising a differential pair; a buffer comprising a second pair ofemitter followers; and a splitter for splitting the input signal intofirst and second differential output signals for output the differentialdriver amplifier output, and the differential signal output,respectively.